North Yemen became independent of the Ottoman Empire in 1918. The British, who had set up a protectorate area around the southern port of Aden in the 19th century, withdrew in 1967 from what became South Yemen. Three years later, the southern government adopted a Marxist orientation. The massive exodus of hundreds of thousands of Yemenis from the south to the north contributed to two decades of hostility between the states. Egypt under Nasser aided the southern Marxist Yemen force militarily, including the use of chemical weapons, specifically mustard gas and perhaps an organophosphate nerve agent, against royalist troops in North Yemen. The two countries were formally unified as the Republic of Yemen in 1990. A southern secessionist movement in 1994 was quickly subdued.